The present invention relates to a fluorometric method and instrument for determining SO.sub.2 concentration, employing a novel base stabilizing mechanism.
The SO.sub.2 molecule has three absorption wavelength ranges in the ultraviolet region, and if excited by ultraviolet rays within any of these wavelength ranges, it generates fluorescent ultraviolet rays. More particularly, a first absorption wavelength range is 340-390 nm, a second is 250-320 nm and a third is 190-230 nm. However, it has been found that in the first wavelength range, the absorption of ultraviolet rays by SO.sub.2 molecules is very weak and that in the second wavelength range, the quenching or masking effect of the nitrogen and O.sub.2 gas in the atmosphere on the absorption and excitation of SO.sub.2 molecules is so high that ultraviolet rays in said second wavelength range cannot be used for satisfactory measurements of SO.sub.2 concentration.
The present invention is concerned with the measurement of SO.sub.2 concentration by using said third wavelength range.
In the third wavelength range, although the background phenomena interfering with the SO.sub.2 excitation are not as strong as in said second wavelength range, still the background offers variations in the zero base indications on a quantitative scale, said variations corresponding to about 0.1-1.6 ppm in terms of SO.sub.2 concentration. To indicate quantitatively only the SO.sub.2 presence, the zero base must be made by a so-called zero gas which ordinarily consists of highly pure N.sub.2, an inert gas comprising any rare gas, or air having SO.sub.2 removed therefrom. In cases where a span gas is used to establish the span base of indications, this span gas should comprise the same component as said zero gas to dilute a known amount of SO.sub.2 to provide the span concentration thereof.